1. Field of the Invention
The present invention relates to reading lights and more particularly a light emitting diode (LED) reading light assembly suitable for use in aviation applications.
2. The Prior Art
It is known to use white LED's as a light source for illumination purposes. For example, U.S. Pat. No. 4,947,291 to McDermott relates to a device for cockpit lighting including a filter for adjustable proportional dimming of the emitted white light from LED's. U.S. Pat. No. 4,963,798 relates to a white LED illumination device which uses the resistance of an incandescent lamp element as a current limiting resistor to protect the LED from excessive current.
It is also known to use a thermally conductive housing to prevent heat from accumulating in the chamber housing the LED lamp. For example, U.S. Pat. No. 5,038,255 to Nishihashi et al. discloses a vehicle lamp having a housing made of synthetic resin to improve heat conductivity, a lens and a plurality of LED's placed on a long light source fixing face member. The light source fixing face is provided on the inner face of the housing so that heat can be effectively disbursed directly to the atmosphere. Similarly, Roney et al, U.S. Pat. No. 5,632,551 discloses an LED vehicle lamp assembly such as for truck trailers or tractors having LED's mounted to a circuit board inside a housing filled with a resin for conducting excess heat to the outside environment. The housing is preferably formed of anodized aluminum to promote heat transfer and to minimize corrosion. See also U.S. Pat. Nos. 5,782,555; 5,785,418 and 5,857,767 to Hochstein. The Hochstein '555 and '418 patents disclose an LED lamp assembly including a thermally conductive plating to conduct heat from the leads of the LED, and a heat sink for conducting heat to the light emitting side of the assembly. The Hochstein '767 patent discloses an LED lamp assembly including an aluminum heat sink formed to include heat radiating fins which dissipate heat.
U.S. Pat. No. 5,765,940 to Levy et al discloses an LED-illuminated stop/tail light assembly for vehicles which includes a lens that is capable of directing light from the LED's both horizontally and vertically. A current regulating assembly mounted within the housing has a first output that provides either a low current level for tail-light mode or a high current level for brake light mode and a second output that provides a current return path. The current regulating assembly maintains steady current through the LED's independent of the power source voltage so as to prevent an overheating of the LED's at higher than normal voltages and to provide consistent non-flickering light output with varying input voltages. The lens includes hyperbolic surfaces and a prismatic lens. See also U.S. Pat. No. 5,580,163 to Johnson, II, which includes a flexible membrane for focusing an LED light source.
Another patent of general interest is U.S. Pat. No. 5,390,092 to Lin which shows a light emitting device receptacle including a housing and a circuit board in the housing to which the LED's are affixed.
A key advantage of LED technology over more conventional light sources, such as incandescent or halogen lamps, is the much higher anticipated life of the LED source. Long lamp life is particularly advantageous in aviation application where lamp replacement is cumbersome and requires the use of highly skilled and certified workers to effect replacement. Current LED's offer mean time between failure values of approximately 100,000 operating hours compared to 1,000 to 4,000 hours for filament based devices, including incandescent and halogen lamps.
Achieving the predicted longevity of the LED's, however, is predicated upon operating them within their prescribed operating envelopes. For LED's, typical maximum full power operating temperatures are limited to 50 degrees Celsius. Above 50 degrees Celsius, they can be operated although at proportionally reduced power up to approximately 85 degrees Celsius, at which point and above they should be at zero power. Although at the present time there are a large variety of LED assemblies, there is still a need for a highly reliable, long life and low heat dissipation LED reading light assembly that produces the maximum light output over the widest range of ambient temperature conditions.